This invention is directed to an automatic fill-stop valve and particularly a valve suitable for stopping the filling of liquid propane tanks when the liquid level therein reaches a predetermined level to prevent tank overfilling.
Liquefied petroleum gas, including propane and butane, is extensively used for household cooking and heating where natural gas or manufactured gas is unavailable. Furthermore, liquefied petroleum gas is used for these purposes on recreational vehicles for these same purposes. Liquefied petroleum gas is extremely volatile and burns readily. In the liquefied state, the vapor pressure of propane and butane is such that the liquid can be stored in tanks of only moderate strength in outdoor locations at the ambient temperatures encountered. These characteristics make liquefied petroleum gas extremely desirable for use as fuels. In addition to cooking and space heating, the gases are employed as internal combustion engine fuel and as process gas.
Liquefied petroleum gas must be handled very cautiously due to its characteristics. For example, liquefied propane and butane have boiling points lower than the normal atmospheric temperatures. Thus, they vaporize readily under normal ambient temperature conditions. The vapor pressure is sufficiently high that they are volatile at ordinary outdoor ambient temperatures, at atmospheric pressure. However, the pressure can be contained in the tank in reasonable thickness so that the volatility provides the tank pressurization which supplies the gas to its point of use.
In addition to the foregoing characteristics, these liquefied petroleum gases have extremely high thermal coefficients of expansion such that even moderate changes in temperature thereof cause the liquid to expand or contract appreciably. For this reason, it is necessary to maintain a void or head space in a tank where the liquefied petroleum gas is stored. A portion of the volume of any such tank must be left unoccupied with the liquid to allow for expansion upon increase in temperature. In fact, most states now prohibit the filling of any such tank with liquefied gas beyond a certain percentage of the volume of the tank.
The most common present device for determining maximum tank fill comprises a level-sensing dip tube which extends down into the tank to the point of highest proper liquid level in the tank. The dip tube vents outside the tank and has a manual valve thereon. The manual valve is opened, and filling is commenced. Vapor from the tank head space is discharged out of the open manual valve and, when the tank is filled with liquid to the bottom of the dip tube, the liquid level is sensed thereat by the liquid discharging from the manual valve. The liquid expands and vaporizes as it reaches the atmosphere, thus creating a telltale white cloud. When seeing such a cloud, the operator shuts off the fill valve and terminates the filling operation.
Of course, such a system requires that the filling operator remain alert, because he must turn off the fill valve at the proper time. If the operator fails to respond at the right time, the tank can be filled considerably above the safe limit. Thereafter, in the event of even a moderate increase in temperature of the liquid, the liquid can expand to fill the entire tank. Further expansion would cause a rapid increase in pressure and could rupture the tank. With tank rupture, an extremely hazardous condition occurs when the volatile liquid and vapor is spilled. A spark can produce a destroying fire.
To prevent the occurrence of such hazardous conditions, it is apparent that an automatic fill-stop valve is required. G. D. Mylander U.S. Pat. No. 3,363,641 illustrates an automatic flow control valve which is responsive to the liquid level so that filling is stopped. This prior automatic flow control valve is arranged so that both the filling flow and the level-sensing flow are automatically shut off, but the price of the end product was too high. Commercial acceptability demanded that the valve be produced at a considerably lower cost without sacrificing quality and reliability. It was not immediately apparent that the automatic shutting off of the level-sensing flow added an unnecessary function which made for a higher valve cost than was necessary to accomplish the basic function.